Hearing or auditory perception is the process of perceiving sounds by detecting mechanical vibrations with a sound vibration input. A human ear has three main components, an outer ear, a middle ear and an inner ear. Each part of the ear serves a specific purpose in the task of detecting and interpreting sound. The outer ear serves to collect and channel sound to the middle ear. The middle ear serves to transform the energy of a sound wave into the internal vibrations of the bone structure of the middle ear and ultimately transform these vibrations into a compressional wave in the inner ear. The inner ear serves to transform the energy of a compressional wave within the inner ear fluid into nerve impulses that can be transmitted to the brain.
The inner ear is the innermost part of the ear, which begins behind an oval window. The oval window, which serves as an intersection between middle ear and inner ear, is a membrane covered opening which receives mechanical vibrations from stapes of the middle ear. The inner ear comprises a bony labyrinth, which is a hollow cavity located in a temporal bone of the skull with two functional parts, a cochlea, and a vestibular system formed by semicircular canals and a vestibule. The bony labyrinth comprises a membranous labyrinth which is in certain locations fixed to a wall of the bony labyrinth and partly separated from the bony labyrinth by the perilymph. The membranous labyrinth contains a liquid, called endolymph. The perilymph is rich in sodium ions and the endolymph is rich in potassium ions, which produces an ionic electrical potential between the perilymph and the endolymph. The unique ionic compositions of the perilymph and the endolymph are adapted for regulating electrochemical impulses of hair cells. Hair cells are cylindrical or flask-shaped cells with a bundle of sensory hairs at their apical end, also called stereocilia. Inner hair cells are arranged on an inner wall and outer hair cells are arranged on an outer wall in specialized areas of the membranous labyrinth. The hair cells are connected to nerve fibres of a vestibulocochlear nerve comprising a vestibular nerve and a cochlear nerve and work as receptor cells of vibratory stimuli of the perilymph and endolymph. The hair cells in the vestibular system serve to sense spatial orientation information and transmit them to a brain via the vestibular nerve generating a sense of balance. One of the specialized areas of the membranous labyrinth is a spiral organ called organ of Corti, which comprises hair cells serving for auditory perception.
The organ of Corti is arranged in the cochlea. The cochlea is a spiral-shaped perilymph filled cavity in the bony labyrinth in contact with the oval window via the vestibule. The spiral of the cochlea typically has 2.5 to 2.75 turns around its axis for a human. The structure of the cochlea includes a scala vestibuli, a scala tympani, a scala media, and a helicotrema. The perilymph filled scala vestibuli joins the perilymph filled scala tympani at the apex of the cochlea, also called helicotrema. On the other end the scala vestibuli is connected to the oval window and the scala tympani is connected to a round window. The round window vibrates with opposite phase to the oval window, e.g. the round window moves out when the stirrup pushes in the oval window and allows movement of the liquid inside of the cochlea. The scala media is located between the scala tympani and the scala vestibuli, separated from the scala tympani by a basilar membrane, and separated from the scala vestibuli by a Reissner's membrane. The scala media contains endolymph. The Reissner's membrane functions as a diffusion barrier for nutrients between perilymph and endolymph. The basilar membrane determines the mechanical vibration propagation properties and is the base of the hair cells containing the steriocilia. The cochlea serves for converting mechanical vibrations received by the ear drum into electrochemical nerve impulses which are then passed to the brain via the cochlear nerve, also called acoustic nerve.
Mechanical vibration stimuli transmitted by the perilymph run through the cochlea and excite the membrane of the membranous labyrinth and the hair cells. Each frequency of the mechanical vibrations received has a specific place of resonance along the basilar membrane of the membranous labyrinth in the cochlea. The movement of the basilar membrane and the mechanical vibrations in the perilymph lead to movement of the stereocilia on the hair cells. The outer hair cells oscillate in cell length with a frequency of an incoming vibration, which serves to amplify incoming mechanical vibrations, called cochlear amplifier. The inner hair cell works as a mechanoreceptor, which produces an electrical signal in response to displacement of the stereocilia of the inner hair cell. The electrical signal originates from a flow of ionic currents across the membrane of the membranous labyrinth through ion channels. The stereocilia of the inner hair cells have a depolarization and a repolarization movement direction. The movement of the stereocilia in depolarization direction leads to increased membrane conductance, allowing more positively charged ions, i.e. potassium and calcium, to pass the membrane and to enter the inner hair cell. Movement in the repolarization direction lowers ionic current. Depolarization of the inner hair cells occurs due to influx of positive ions resulting in a receptor potential, which opens voltage-dependent calcium channels (VDCC). Calcium ions can enter the cell through the voltage-dependent calcium channels (VDCC) and trigger the release of neuro-transmitters which connect to receptors of the fibres, also called axons, of the cochlear nerve connected to the hair cells resulting in increased firing, i.e. emission of electrochemical nerve impulses. Repolarization of the hair cells occurs due to low concentration of positive ions in the perilymph in the scala tympani, which leads to an electrochemical gradient and a flow of positive ions through ion channels to the perilymph. The electrochemical nerve impulses are transmitted to the brain via the cochlear nerve. The brain processes the nerve impulses received by all hair cells and the spatial-temporal pattern of nerve impulses resulting from the firing of different hair cells in auditory perception.
Healthy human ears are able to hear sounds in a frequency range of 0.012 kHz to 20 kHz and have a sensitivity peak in a range between 1 kHz to 5 kHz. The human ear can resolve frequency differences down to 3.6 Hz, allowing humans to differentiate between two sounds with a difference as small as 3.6 Hz. With age the hearing deteriorates, called Presbycusis or age-related hearing loss, which leads to a lower audible hearing frequency range. Most adults are unable to hear high frequencies above 16 kHz. The cause of age-related hearing loss is typically a sensorineural hearing loss.
The hearing can be considered impaired, if one or more of the functions of one or both ears of a human are impaired. A hearing impairment can be classified as sensorineural hearing loss, conductive hearing loss or a combination of the two called mixed hearing loss.
The most common kind of hearing impairment is the sensorineural hearing loss, which results from impairment of the vestibulocochlear nerve, the inner ear, and/or central processing centers of the brain. A majority of sensorineural hearing loss is caused by dysfunction of the hair cells of the organ of Corti in the cochlea leading to decreased hearing sensitivity. The hair cells can be dysfunctional at birth or damaged, e.g., due to noise trauma, infection, long time noise exposure, or genetic predisposition. Often the outer hair cells, which are particularly sensitive to damage from exposure to trauma from overly-loud sounds or ototoxic drugs, are damaged and the amplification effect is lost.
Cochlear hearing loss which is a representative phenomenon of hearing impairment is related to damage to the hair cells of the cochlea, and this damage causes hearing loss in the following two forms.
A first form is damage to the outer hair cells, which is the cause of most sensorineural hearing loss. Due thereto, the active mechanism of the cochlea is damaged, so that the motion of a basilar membrane decreases compared to that of a normal state, with the result that frequency selectivity decreases. A second form is damage of inner hair cells. This may result in a decrease in the efficiency of signals transferred to a primary auditory cortex. In particular, speech recognition ability is greatly decreases, and the ability to discriminate signals from noise is further deteriorated in the presence of noise.
A region in which the inner hair cells are substantially damaged and in some cases completely lost and do not perform their own functions is called a cochlear dead region (DR). The cochlear dead region exhibits characteristics where the inner hair cells and nerves of the inside thereof do not induce nervous activities in response to stimuli falling within the range of relevant characteristic frequencies (CFs), and then relevant acoustic stimulus information is not transferred to a primary auditory cortex.
Some of the hearing impairments can be treated surgically. A major part of humans with a hearing impairment, however, has to rely on devices which improve hearing, so called hearing aid devices. Hearing aid devices are used to stimulate the hearing of a user, e.g., by sound generated by a speaker, by bone conducted vibrations generated by a vibrator of a bone anchored hearing aid, or by electric stimulation impulses generated by electrodes of a cochlear implant. Hearing aids can be worn on one ear, i.e. monaurally, or on both ears, i.e. binaurally. Binaural hearing aid devices comprise two hearing aids, one for a left ear and one for a right ear of the user. The binaural hearing aids can exchange information with each other wirelessly and allow spatial hearing.
Hearing aids typically comprise a microphone, an output transducer, e.g. speaker or vibrator, electric circuitry, and a power source, e.g., a battery. The microphone receives a sound from the environment and generates an electrical sound signal representing the sound. The electrical sound signal is processed, e.g., frequency selectively amplified, noise reduced, adjusted to a listening environment, and/or frequency transposed or the like, by the electric circuitry and a processed sound is generated by the output transducer to stimulate the hearing of the user. Instead of an output transducer the cochlear implant typically comprises an array of electrodes, which are arranged in the cochlea to stimulate the cochlear nerve fibres with electric stimulation impulses. In order to improve the hearing experience of the user a spectral filterbank can be included in the electric circuitry, which, e.g., analyses different frequency bands or processes electrical sound signals in different frequency bands individually and allows improving the signal-to-noise ratio. Spectral filterbanks are typically running online in any hearing aid today.
Typically the microphones of the hearing aid device used to receive the incoming sound are omnidirectional, meaning that they do not differentiate between the directions of the incoming sound. In order to improve the hearing of the user, a beamformer can be included in the electric circuitry. The beamformer improves the spatial hearing by suppressing sound from other directions than a direction defined by beamformer parameters, i.e. a look vector. In this way the signal-to-noise ratio can be increased, as mainly sound from a sound source, e.g., in front of the user is received. Typically a beamformer divides the space in two sub-spaces, one from which sound is received and the rest, where sound is suppressed, which results in spatial hearing.
For certain acoustical environments, a microphone to record direct sound can be insufficient to generate a suitable hearing experience for the hearing aid device user, e.g., in a highly reverberant room like a church, a lecture hall, a concert hall or the like. Therefore hearing aid devices can include a second input for sound information, e.g., a telecoil or a wireless data receiver, such as a Bluetooth receiver or an infrared receiver, or the like. When using telecoil or other wireless technology an undistorted target sound, e.g., a priest's voice in a church, a lecturer's voice in a lecture hall, or the like is available directly in the hearing aid by wireless sound transmission.
One way to characterize hearing aid devices is by the way they are fitted to an ear of the user. Conventional hearing aids include for example ITE (In-The-Ear), ITC (In-The-Canal), CIC (Completely-In-the-Canal) and BTE (Behind-The-Ear) hearing aids. The components of the ITE hearing aids are mainly located in an ear, while ITC and CIC hearing aid components are located in an ear canal. BTE hearing aids typically comprise a Behind-The-Ear unit, which is generally mounted behind or on an ear of the user and which is connected to an air filled tube or a lead that has a distal end that can be fitted in an ear canal of the user. Sound generated by a speaker can be transmitted through the air filled tube to an ear drum of the user's ear canal or an electrical sound signal can be transmitted to an output transducer arranged in the ear canal via the lead.
Nearly all hearing aids have at least one insertion part, which is adapted to be inserted into an ear canal of the user to guide the sound to the ear drum. Inserting the insertion part of a hearing aid device into an ear canal that transmits device generated sound into the ear canal can lead to various acoustic effects, e.g., a comb filter effect, sound oscillations or occlusion. Simultaneous occurrence of device generated and natural sound in an ear canal of the user creates the comb filter effect, as the natural and device generated sounds reach the ear drum with a time delay. Sound oscillations generally occur only for hearing aid devices including a microphone, with the sound oscillations being generated through sound reflections off the ear canal to the microphone of the hearing aid device. A common way to suppress the aforementioned acoustic effects is to close the ear canal, which effectively prevents natural sound to reach the ear drum and device generated sound to leave the ear canal. Closing the ear canal, however, leads to the occlusion effect, which corresponds to an amplification of a user's own voice when the ear canal is closed, as bone-conducted sound vibrations cannot escape through the ear canal and reverberate off the insertion part of the hearing aid device. To reduce the occlusion effect the insertion part of the hearing aid device can be inserted deeper into the ear canal to adhere to the bony portion of the ear canal and to seal the ear canal.
Hearing aid devices can be further improved, when the hearing impairment of the user is exactly known, e.g., by allowing an adjustment of hearing aid parameters to the hearing impairment of the user. Knowing the frequency range, for which a user has a reduced hearing ability allows for example to shift frequencies or amplify certain frequency ranges to allow for a better hearing experience of the user.
During processing of incoming sound in a hearing aid the amplification of frequencies in the frequency range corresponding to dead regions of the user is typically not beneficial and can impair speech intelligibility.
In conventional hearing care the cochlear dead region phenomenon is largely ignored due to the difficulty in establishing the existence and frequency region of the cochlear dead regions.
In B. C. Moore, M. Huss, D. A. Vickers, B. R. Glasberg, and J. I. Alcántara, “A Test for the Diagnosis of Dead Regions in the Cochlea” British Journal of Audiology, 34, 205-224 (2000) a method using a threshold equalizing noise (TEN) test to determine cochlear dead regions is presented. The TEN test stimulates the hearing of a user using a so called “threshold equalizing noise”, which is spectrally shaped so that, for normally hearing users, it would give equal masked threshold for pure tone signals over all frequencies within a range of 0.25 kHz to 10 kHz. A level of the threshold equalizing noise is specified as a level in a one-ERB, equivalent rectangular bandwidth, (132 Hz) wide band centred at 1000 Hz. A pure tone is used with the threshold equalizing noise to stimulate the hearing of a user. The threshold equalizing noise reduces the off-frequency listening of the pure tone and therefore allows testing for cochlear dead regions. A frequency region with living inner hair cells leads to a detection of a signal with characteristic frequencies close to the frequency region and the threshold in the TEN is close to that for normal-hearing users. A dead region leads to a detection of characteristic frequencies different from that of the signal frequency and the threshold in the TEN is higher than normal.
WO 2012/081769 A1 presents an apparatus and a method for detecting a cochlear dead region. The apparatus comprises a control unit with a stimulus generation unit and an Acoustic Change Complex (ACC) measurement unit. The stimulus generation unit is configured to generate stimuli, which are provided to a user via a headphone and the Acoustic Change Complex measurement unit is configured to measure Acoustic Change Complexes depending on the stimuli. The Acoustic Change Complex is similar to a P1-N1-P2-complex and represents a cortical response due to change in the acoustic features of the stimuli. The P1-N1-P2-complex is a portion of cortical auditory evoked potential, which is a prior art test method to determine the degree of hearing of a user.
There exists a need for reliably identifying the cochlear dead region, and preferably offering schemes that allow for accounting for the determined dead regions in order to enhance hearing perception for a hearing impaired.